In the production of zirconium and hafnium, the Kroll process chiefly concerns the reduction of the tetrahalides of zirconium or hafnium with magnesium to form zirconium or hafnium metal in accordance with the formula. EQU ACl.sub.4 + 2 Mg .fwdarw. 2 MgCl.sub.2 + A
(wherein A represents zirconium or hafnium.)
The zirconium or hafnium sponge as provided by the Kroll process contains a substantial amount of excess magnesium, which is required in the reaction, and magnesium chloride by-product of the reaction. Relatively extensive purification procedures e.g. complex high temperature vacuum distillations and the like, are required to obtain the metal sponge product, free from magnesium chloride and magnesium, in pure form. As a serious factor in the economics of the process, the magnesium is not presently recovered in reusable or salable quality, and must be further processed, discarded or disposed of at distress prices.
Salt tapping has been practiced for many years in the Kroll reduction of titanium tetrachloride with magnesium. However, there are major differences between zirconium, for example and titanium, making salt tapping as conventionally practiced in the Kroll reduction of titanium inapplicable for zirconium. With titanium the salt taps are normally conducted intermittently, usually three or four taps per run, and bottom tapping is universal. Upon reduction, the titanium sponge forms in a dendritic manner, generally completely filling the reactor with a loosely defined mass of small crystals. When the magnesium chloride salt is drained away, the titanium sponge is exposed to the entering titanium tetracloride and probably reacts to form partially reduced forms (TiCl.sub.2, TiCl.sub.3) which subsequently pose no particular problem, indicating that conversion to a more stable form (Ti or TiCl.sub.4) takes place during further processing.
Many attempts have been made to tap molten magnesium chloride during zirconium reduction, although techniques used for titanium could not be used with zirconium because metallic zirconium, being heavier than titanium settles compactly like a dense mud on the bottom of the crucible. Also it has been found that if any zirconium is exposed to the tetrachloride vapor, and some usually is when the magnesium salt is drained from the crucible, partially reduced forms of zirconium are produced. While such forms apparently are stable under conditions of reduction, and even survive the distillation operation, upon exposure to air after distillation such partially reduced forms of zirconium will spontaneously ignite and normally will set the entire batch of zirconium sponge on fire, posing an obvious problem.